Experimental investigation of undrained and drained shear behavior of fat clay stabilized with industrial sewage sludge ash and reinforced with PVA fibers

The mechanical properties of industrial sewage sludge ash (ISSA) stabilized and PVA fiber-reinforced fat clay were studied with special reference to the trade-off between strength improvement and brittleness enhancement by stabilization. The experimental program included a complete set of Atterberg limits, standard Proctor compaction, unconsolidated-undrained (UU) triaxial, consolidated-drained direct shear tests, scanning electron microscopy (SEM), and toxicity characteristic leaching procedure (TCLP) analysis. In the study, the effects of ISSA content (0–15%), PVA fiber dosage (0-0.5%), curing period (14–42 days), and moisture condition were examined. The results show that the ISSA stabilization significantly improved the shear strength of the fat clay and also significantly increased its brittleness. Under a confining pressure of 400 kPa, the maximum deviatoric stress increased from 553 kPa for untreated clay to 1602 kPa for soil stabilized with 5% ISSA. Increasing the ISSA content to 10% resulted in only limited additional improvement. The PVA fiber reinforcement improved the post-peak strength, the deformation capacity, and the resistance to shear localization. Within the investigated range, 5% ISSA, 0.3% PVA fibers, and 42 days of curing provided the most balanced performance. SEM observations showed particle aggregation, structural modification, and stronger interaction between fibers and matrix. TCLP results showed that heavy-metal concentrations were below regulatory limits for the tested conditions. The results suggest that ISSA stabilization with PVA fiber reinforcement is a promising method to improve high-plasticity clay and to reduce the brittleness caused by cementitious stabilization.